The subject of the proposed research is the study of the ontogeny (origins, differentiation pathways and anatomical distribution) and functions of the lymphocytes which comprise the T cell system in the rat. Central to this study is the use of highly specific antilymphocyte sera in conjunction with cell separation techniques to identify and isolate developmentally and functionally distinct lymphocyte subpopulations. These antisera have been used thus far to identify pluripotent lymphohemopoietic stem cells, prothymocytes, 4 subsets of cortical thymocytes and 2 subsets of peripheral T cells. Using these markers, we now propose to undertake a series of in vivo and in vitro studies to trace the developmental pathway of prothymocytes from pluripotent stem cells. We also plan to determine whether stem cells from embryonic and adult rats have different developmental potentialities and/or proliferative capabilities, as suggested by results of preliminary experiments. Lastly, we plan to study the functional attributes of antigenically distinct subsets of T cells in spleens of rats of different ages, with a view toward identifying a population of suppressor cells. BIBLIOGRAPHIC REFERENCES: Goldschneider, I. (1976). Antigenic relationship between bone marrow lymphocytes, cortical thymocytes and a subpopulation of peripheral T cells in the rat: Description of a bone marrow lymphocyte antigen. Cell Immunol. 24: 289-307. Goldschneider, I., Bollum, F.J., Gregiore, K., and Barton, R.W. (1977). Demonstration of terminal deoxynucleotidyl transferase in thymocytes by immunofluorescence. Proc. Nat. Acad. Sci. USA (in press).